1. Field of the Invention
This invention relates generally to a technique for controlling the relative humidity of a cathode input airflow to a fuel cell stack and, more particularly, to a technique for controlling the relative humidity of a cathode input airflow to a fuel cell stack that includes controlling valves at the cathode output of the stack in a non-linear manner so that a portion of the cathode exhaust can be selectively directed through a water vapor transfer unit without affecting the pressure of the stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The membranes within a fuel cell stack need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. If the wetness of the membranes is not at or near an optimum level, then the durability of the membranes is reduced. Therefore, to help maintain membrane relative humidity, it is known in the art to humidify the airflow to the cathode side of the stack.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust from the stack will include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust, and use the separated water to humidify the cathode airflow.
FIG. 1 is a schematic diagram of a fuel cell system 10 that humidifies a cathode input airflow to a fuel cell stack 12 in this manner. A compressor 14 provides a compressed airflow on line 16 to the cathode side of the stack 12. A humidified cathode exhaust gas is provided on line 18 at the output of the cathode side of the stack 12. The airflow from the compressor 14 on the line 16 is directed through one side of a WVT unit 20 and the cathode exhaust gas on the line 18 is directed through another side of the WVT unit 20. The WVT unit 20 includes permeation membranes, or other porous materials, as is well understood in the art, that collects water vapor and liquid water in the cathode exhaust gas and uses this water to humidify the airflow to the cathode input.
The relative humidity of the cathode input airflow and the pressure within the fuel cell stack 12 need to be tightly controlled for proper fuel cell stack operation. The mass flow of the water transferred through the WVT unit 20 depends on the partial pressure of the water, the flow of air through the WVT unit 20 and other system parameters. To control the relative humidity and pressure, proportional control valves 22 and 24 are selectively opened and closed to direct the cathode exhaust gas through the WVT unit 20 or by-pass the WVT unit 20 on a by-pass line 26. A dew-point sensor 28, or other suitable sensor, measures the relative humidity of the cathode airflow after the WVT unit 20, and that value in combination with the stack operating conditions, such as current density, temperature, pressure, etc., determines the position of the control valves 22 and 24 so that the proper relative humidity is provided for the cathode input air. A pressure sensor 30 measures the pressure in the line 16 at the cathode inlet to the stack 12.
In one embodiment, the proportional control valves 22 and 24 are butterfly valves, well known to those skilled in the art. FIG. 2 is a cross-sectional view of a butterfly valve 32 that can be used for the control valves 22 and 24. The butterfly valve 32 includes a rotatable valve plate 34 that selectively opens and closes a flow path 36 to provide the flow control.
The control valves 22 and 24 control the humidity of the cathode input airflow and the cathode stack pressure. If the controller of the fuel cell system 10 wants to change the pressure and maintain a certain relative humidity, or change the relative humidity and maintain the pressure, separate humidity and pressure controllers are used to control the valves 22 and 24. FIG. 3 is a graph with controller output percentage on the horizontal axis and plate angle on the vertical axis showing the relationship between the angle of the valve plates for the valves 22 and 24 to provide relative humidity control. The known algorithms used for relative humidity control maintain the orientation of the valve plate for the control valve 24 on line 40 and the orientation of the valve plate for the control valve 22 on line 42 so that when the plate angles are added they will equal 90°. Therefore, the sum of the angle of the two plates for the valves 22 and 24 is constant.
FIG. 3 shows that there is a linear relationship between the opening and closing of the valve plates of the valves 22 and 24. However, the opening and closing of the valve plates is actually non-linear. What this means is that if there is a displacement to change the flow distribution through the valves 22 and 24, the total resistance provided by the valves 22 and 24 will change because of the non-linear characteristics of the valves 22 and 24 (see FIG. 4), which will cause the pressure in the stack 12 to change. Therefore, the pressure controller will have to adjust the position of the plate for the valve 22 to correct the change in the system pressure. Thus, the pressure controller influences the humidity controller, which could cause system instability.